This invention relates to recovering viscous petroleum from petroleum-containing formations. Throughout the world there are several major deposits of high-viscosity crude petroleum in oil sands not recoverable in their natural state through a well by ordinary production methods. In the United States, the major concentration of such deposits is in Utah and California. By far the largest deposits in the world are in the Province of Alberta, Canada. The depths of these deposits range from surface outcroppings to about 2000 feet.
To date, none of these deposits has been produced commercially by an in-situ technology. Two commercial mining operations exist, and they are in a shallow Athabasca deposit and others are proposed. There have been many in-situ well-to-well pilots, all of which used some form of thermal recovery after establishing communication between an injector well and a producer well. Normally such communication has been established by introducing a pancake fracture. The displacing or drive mechanism has been steam and combustion or steam and chemicals. Another proposal is to develop well-to-well communication by injecting steam over a period of several years into a high water saturation zone underlying the tar sand deposit at a depth of round 1800 feet. Probably the most active in-situ pilot in the oil sands uses the huff-and-puff single-well method of steam stimulation. This pilot has been producing about 5000 barrels of viscous petroleum per day for several years from about 50 wells.
The most difficult problem in any in-situ well-to-well viscous petroleum project is establishing and maintaining communication between injector and producer. In shallow deposits, fracturing to the surface has occurred in a number of pilots whereupon satisfactory drive pressure could not be maintained. In many cases, problems arise from healing of the fracture when the viscous petroleum that had been mobilized through heat cooled as it moved toward the producer. The cool petroleum is essentially immobile, since its viscosity in the Athabasca deposits, for example, is on the order of 100,000 to 1,000,000 cp at reservoir temperature.
As noted, the major problem of the economic recovery from many formations has been establishing and maintaining communication between an injection position and a recovery position in the viscous oil-containing formation. This is primarily due to the character of the formations, where effective mobility of fluids may be extremely low, and in some cases, such as the Athabasca Tar Sands, virtually nil. Thus, the Athabasca Tar Sands, for example, are strip mined where the overburden is limited. In some tar sands, hydraulical fracturing has been used to establish communication between injectors and producers. This has not met with uniform success. A particularly difficult situation develops in the intermediate overburden depths, which cannot stand fracturing pressure.
Heretofore, many processes have been utilized in attempting to recover viscous petroleum from viscous oil formations of the Athabasca Tar Sands type. The application of heat to such viscous petroleum formations by steam or underground combustion has been attempted. The use of vertical slotted liners positioned in the viscous oil formation as a conduit for hot fluids has also been suggested. However, these methods have not been overly successful because of the difficulty of establishing and maintaining communication between the injector and the producer. Clearly, if one could establish and maintain communication between injector and producer, regardless of the drive fluid or recovery technique employed, it would open up many of these viscous petroleum deposits to a number of potentially successful projects.